If a card says something like, "Action: When this unit attacks target opponent takes 2 indirect damage." can the target of the indirect damage be a player other than the target of the attack that triggered the action in the first place? For example, the indirect damage might go to yourself or a third player in a multiplayer game.

Any help appreciated. thanks!

If it says "target player" any player, including you, can be the target. If it says "target opponent", any other player can be the target, but not you. You are not your opponent after all. In your example, you could attack opponent A, and target opponent B with the effect, but you couldn't target yourself with it.

Keep in mind that there are no official multiplayer rules though, so sometimes you'll have to use your best judgement and common sense when you run into questionable situations in multiplayer games.

I'm sorry, I should have said "player" instead of "opponent" in my example. Thank you for your response. Very helpful!